The "perfect" binding of books with hot melt adhesives is a high speed, continuous operation used in the book-binding industry for the binding of paper-back books, catalogs, magazines, etc. In this operation, the books are printed in sheets or long continuous webs which are cut, folded and properly arranged to form a series of stacks, referred to as signatures, which are then fed into a revolving mechanism equipped with clamps. The folded edges of the clamped sheets are first cut to remove the signature folds, and then the edges of the sheets are roughened so as to expose a maximum amount of fibers and thereby render them more receptive to the hot melt adhesive composition. A layer or coat of the molten adhesive composition is then applied by means of a suitable applicator, after which a paper backing or other suitable cover is applied before the molten adhesive sets to form the spine of the book. The perfect bound book is then trimmed to the desired size, and optionally the exposed sheet edges are colored as by painting or dyeing to give the finished product.
Hot melt adhesive compositions useful for the perfect binding of books serve at least two critical functions. In the first place, they must hold the book together during its formation. Thus, it is necessary that they be capable of forming thin films in the molten state; that they wet rapidly; that they have good tack or adhesion for similar and dissimilar sheets ranging from coated paper, enameled stock, cross and long grain, coated or uncoated sheets, groundwood or sulfite; that they be quick setting but not so quick as to prevent the cover from bonding; and that they be capable of exhibition a required range of viscosities in order to be applicable for use with any available and generally suitable hot melt applicator. Secondly, on cooling or setting, the hot melt adhesive must be able to produce spines having good "wear potential" so as to be capable of preventing the book from falling apart. Wear potential includes the properties of high bonding strength, flexibility, high film strength (tensile strength) and resistance to aging, mold growth, and "cold crack." "Cold crack" is the undesirable hardness of the spine which prevents the opening of a book to 360.degree., i.e. so that the covers are back-to-back to one another, without causing the spine to split. This characteristic is dependent upon temperature and is of particular importance when books are exposed to cold climates. Since the perfect bound book has no staples, threads, stitches, etc., cracking of the spine causes the book to split.
An additional requirement of many publishers is the feature that the book or magazine be "easy-opening", i.e., that the two-page spread may be completely opened and will remain in the open position when placed down, as on a table or the like. This characteristic is in direct contrast to the behavior exhibited by conventional stiff paperback books which will snap closed as soon as they are placed down (known in the trade as "mousetrapping"). These books can normally be kept open only by exerting pressure directly on the spine so as to loosen, break or crack the adhesive binding.
The conventional adhesives are often limited in their effectiveness as hot melts for the perfect binding of books because they are only mediocre with regard to one or more of the properties enumerated above. Often it has been necessary to compromise certain properties in order to obtain products having at least a narrow range of useful properties. Thus, for example, hot melts based on polyvinyl acetate have shown poor warm flow (i.e. undesirable softness) and high cold crack characteristics. In addition, hot melts based on ethylene-vinyl acetate copolymer systems have experienced various adhesive problems and the books which have been bound with the latter hot melts have exhibited poor "easy-opening" properties. U.S. Pat. No. 3,837,994 teaches a hot melt adhesive which overcomes the previously described problems but which has been found to be limited in its commercial applicability due to a primer step being required when the adhesive is to be used on coated stock in conjunction with the high speed equipment found in the many operating plants. The use of this primer in order to obtain superior properties is a drawback, particularly in the case of magazines which are issued weekly where, in order to maintain the large scale production and widespread circulation requirements, multiple plants (often with multiple lines) are simultaneously run. It will be appreciated that the multiplication of equipment required if primers are to be used (primer applicators, etc.) is constrained both from an economic as well as a physical limitation.
It is therefore an object of the present invention to provide a method for the perfect binding of books which does not require the use of a primer even for coated stocks. Another object is to provide such a method which does not require the user to compromise any of the useful characteristics and especially the easy opening and cold crack characteristics. It is also an object of the invention to provide a novel hot melt adhesive for use in such a process.